The present invention relates to a polymer foam having fine cells and excellent heat resistance, a process for producing the same and a foam substrate. The polymer foam is exceedingly useful as, for example, an internal electrical insulator, cushioning material or heat insulator for electronic appliances and others, and the foam substrate is useful as a circuit substrate.
Conventional general processes for foam production include chemical processes and physical processes. In the chemical processes, a compound (blowing agent) added to a polymer base is thermally decomposed and cells are formed by the resultant gas to obtain a foam. However, this foaming technique has a disadvantage that after the gas generation, a residue of the blowing agent tends to remain in the foam. This technique therefore poses a problem concerning fouling by corrosive gases or impurities especially when the foam is used as an electronic part or the like because fouling prevention is highly required in such applications.
On the other hand, a general physical process comprises dispersing a low boiling liquid (blowing agent) such as a chlorofluorocarbon or hydrocarbon into a polymer and then heating the polymer to volatilize the blowing agent, thereby forming cells. For example, U.S. Pat. No. 4,532,263 discloses a method for obtaining a foamed polyether imide or another foamed polymer using methylene chloride, chloroform, trichloroethane or the like as a blowing agent. However, this foaming technique has problems concerning the harmfulness of the substances used as a blowing agent and various influences thereof on the environment, including ozonosphere depletion. In addition, it is difficult to obtain with this technique a foam having fine cells uniform in diameter, although the technique is generally suitable for obtaining a foam having a cell diameter of several tens of micrometers or larger.
Recently, a technique for obtaining a foam having a small cell diameter and a high cell density was proposed which comprises dissolving a gas such as nitrogen or carbon dioxide in a polymer at a high pressure, subsequently releasing the polymer from the pressure, and heating the polymer to a temperature around the glass transition temperature or softening point of the polymer to thereby form cells. This foaming technique, in which nuclei are formed in a thermodynamically unstable state and are allowed to expand and grow to thereby form cells, has an advantage that a foam having a finely cellular novel structure is obtained. For example, application of this technique to a styrene resin having a syndiotactic structure is disclosed in JP-A-10-45936 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d). Specifically, this reference discloses a method for obtaining a molded foam having closed cells with a cell size of from 0.1 to 20 xcexcm. There is a description therein to the effect that this molded foam is useful as an electric circuit member. However, this molded foam deforms or bends when used at temperatures not lower than 100xc2x0 C., because styrene resins having a syndiotactic structure generally have a glass transition point around 100xc2x0 C. Consequently, applications of this molded foam are limited to a narrow range.
JP-A-6-322168 discloses a method which comprises heating a pressure vessel containing a thermoplastic polymer, e.g., a polyether imide, to or around the Vicat softening point of the polymer, impregnating the heated polymer with a gas in a supercritical fluid state, and then releasing the polymer from the pressure to obtain a porous foamed article having a low density. However, this method has the following drawback. Since the polymer is heated to or around the Vicat softening point thereof for impregnation with a gas in a high-pressure vessel, the gas readily expands upon pressure decrease because the polymer is in a molten state. The resultant foam hence has a cell size as large as about from 10 to 300 xcexcm. In the case where this foam is laminated with a metal foil to produce a laminate for use as a circuit substrate, pattern formation on the metal foil side by etching is limited in the degree of pattern fineness. In addition, the foam is expected to further have a problem that chemicals used for the processing, such as a resist, an etchant, and a stripping fluid, infiltrate into pores of the foam to considerably reduce electrical reliability.
Accordingly, an object of the invention is to provide a heat-resistant polymer foam having excellent heat resistance, a fine cellular structure, and a low relative density.
Another object of the present invention is to provide a process for producing the heat-resistant polymer foam.
Still another object of the invention is to provide a foam substrate which has a metal foil where a fine pattern can be formed and which is useful as a circuit substrate having high electrical reliability.
As a result of intensive studies to accomplish the above objects, it has been found that a foam having excellent heat resistance and exceedingly fine cells is obtained by impregnating a heat-resistant polymer with a non-reactive gas such as carbon dioxide under pressure, reducing the pressure, and then heating the polymer at a specific temperature. The present invention has been completed based on this finding.
The present invention provides a heat-resistant polymer foam which comprises a heat-resistant polymer and has an average cell diameter of from 0.01 xcexcm to less than 10 xcexcm. The heatresistant polymer has a glass transition point of, e.g., 120xc2x0 C. or higher. This heat-resistant polymer includes a polyimide, a polyether imide and the like.
The present invention further provides a process for producing a heat-resistant polymer foam which comprises impregnating a heat-resistant polymer with a non-reactive gas under pressure, reducing the pressure, and then heating the impregnated polymer at a temperature exceeding 120xc2x0 C. to foam the polymer. The heating for foaming is preferably conducted at a temperature at which the heat-resistant polymer in an unfoamed state has a modulus of elasticity of 1xc3x97107 Pa or higher. The heat-resistant polymer to be foamed has a glass transition point of, e.g., 120xc2x0 C. or higher. The heat-resistant polymer may be, e.g., a polymer selected from polyimides and polyether imides. The non-reactive gas is, for example, carbon dioxide. The heat-resistant polymer may be impregnated with the non-reactive gas in a supercritical state.
The present invention also provides a foam substrate which comprises a foamed resin layer comprising the heat-resistant polymer foam and a metal foil layer disposed on at least one side of the resin layer.